1. Field of the Invention
The invention relates broadly to prosthetic implants, and more particularly, to prostheses for human joints, such as the knee, implantable by means of arthroscopic as well as open surgical techniques.
2. Discussion of the Prior Art
Previous proposals for artificial knee prostheses including components for surgical implantation into a patient's knee are known in the art. The complexity of normal knee movement, however, has rendered the attainment of natural knee action quite difficult. More specifically, the natural knee joint includes the bottom part of the femur, constituted by the two condyles, the lower parts of which bear upon the complementary shaped upper surface plateaus of the tibia through the intermediary of cartilage or meniscus. Connection through the knee is provided by means of ligaments which also provide joint stability and assist in absorbing stresses applied to the knee. The femur, cartilage and tibia are normally subjected to significant compression loading in supporting the weight of the body.
Movement of the normal knee is not a true hinged joint about a single center but, rather, is a complex action including rocking, gliding and axial rotation. During the first part of the knee movement from full extension of the leg towards flexion, there is pivotal rotation of the tibia about the femur, which is then converted to a rocking movement wherein the femoral condyles roll posteriorly on the tibial plateaus. The rocking movement then changes to a combined sliding and pivoting movement wherein successive points on the femoral condyles slide forward on the tibial plateaus until full flexion is obtained. In other words, the flexion movement is polycentric, that is, about different centers which are not fixed in one position but lie in a somewhat spiral or polycentric pathway.
A variety of total knee prostheses have been proposed, essentially being of two broad types, hinged and non-hinged. Knee prostheses of the first category possess significant disadvantages in that they generally involve the removal of natural ligaments and only permit motion about a single axis as opposed to the controlled rotation and translation characteristic of a natural, healthy knee.
Knee prostheses of the second type generally include femoral components secured to the condylar surfaces of the femur, typically having cylindrical bearing surfaces, and tibial components fixed to the tibial plateaus, the femoral components bearing against the upper surfaces of corresponding tibial components. Examples of prostheses of the latter type are shown in U.S. Pat. No. 4,470,158 to Pappas et al; U.S. Pat. No. 4,211,228 to Cloutier; U.S. Pat. No. 4,207,627 to Cloutier; and U.S. Pat. No. 3,953,899 to Charnley.
In addition to total knee replacement, unicompartmental knee replacement is known wherein a single compartment of the knee is surgically restored. Typically, the medial or lateral portion of the tibio-femoral joint is replaced without sacrificing normal remaining structure in the knee. For instance, U.S. Pat. No. 4,340,978 to Buechel et al discloses a unicompartmental knee replacement device including a tibial platform secured to the tibia and having a track for receiving a bearing insert. A femoral component is attached to one of the condylar surfaces of the femur and is provided with a generally convex spherical inferior surface for engaging the superior surface of the bearing insert. Similar unicompartmental knee implants are shown in U.S. Pat. No. 4,743,261 to Epinette; U.S. Pat. No. 4,309,778 to Buehel et al; U.S. Pat. No. 4,193,140 to Treace; U.S. Pat. No. 4,034,418 to Jackson et al; and U.S. Pat. No. 3,852,830 to Marmor.
The non-hinged knee implants previously discussed, while possessing advantages over the hinged devices, nonetheless are characterized by numerous drawbacks. Many of the prior art prostheses require the removal of a great deal of bone from the femur and tibia in order to accommodate the implant, thus complicating and prolonging the surgical procedure and reducing the amount of bone available in reserve should subsequent restorative measures be required. Additionally, alignment of the prosthesis components is extremely difficult, and even small misalignments lead to an imbalance of the forces transmitted from the femoral component to the tibial component. The asymmetric distribution of load on the plateaus of the tibial component can result in tibial loosening and failure of the prosthesis. Moreover, inadequate fixation of the prosthesis can occur, possibly resulting in the prosthesis twisting loose from the implanted position.
The misalignment and anchoring problems associated with conventional prostheses are due in part to the fact that the prosthesis is secured in place by means of cement applied to the prothesis after a trial fit and prior to actual fixation. Although the joint may have been precisely prepared to accept the prosthesis, and although the femoral and tibial components may have been accurately aligned during trial fitting, deviation from the desired location is apt to occur when the prosthesis is removed to place cement on the prepared bone surface and then replaced on the bone surface.
The prior art prosthetic devices have another disadvantage in that excess cement tends to escape from between the bone and the implant around the edges of the implant. The excess cement, if not removed, may deteriorate and crumble, thereby becoming a source of possible irritation. Additionally, cracking and breakage of the cement may lead to loosening of the cement bond, thus jeopardizing the integrity of the cemented parts. Therefore, additional steps are typically undertaken to remove the excess cement squeezed out during the surgical procedure.
Several of the prior art prosthetic devices previously referred to are illustrative of the foregoing deficiency. For example, Buechel et al ('978) is directed to a unicompartmental knee prosthesis wherein the prosthesis components must be removed after a satisfactory trial fit to allow cement to be placed on the bone surfaces. The components are then reintroduced into the surgical site, located in the pre-established position and firmly held in compression with the bone until complete polymerization has been obtained. Excess cement is removed from the edges of the prosthetic component by a scalpel and curette. Similarly, Charnley teaches inserting cement through a hole cut into the head of the tibia. The anterior end of the tibial component is then elevated to cause the posterior end to press into the tibia bone so as to close the posterior route of escape for the cement. Treace discloses a knee prosthesis for fixation to the femur including a curved body provided with a plurality of cement holding rings fixedly attached to and extending upwardly from the upper surface of the body. The femur must be prepared by drilling slots therein for receiving the cement holding rings subsequent to cement being injected into the slots.